1. Field of Invention
The present invention relates to a method of manufacturing a film printed circuit board, which is used in a chip on flex or chip on film process. More particularly, the present invention relates to a method of manufacturing a copper structure on a film printed circuit board, which is used in a chip on flex or chip on film process.
2. Description of Related Art
Driving integrated circuit (IC) packaging technology has evolved from a tape automatic bonding (TAB) process into a chip on flex or chip on film (COF) process. The driving force behind the evolution is the demand for increasingly smaller displays with higher dpi resolutions, while the sizes of the machines for which the displays are adapted are always decreasing.
The COF process makes a flip chip bond on a flexible or film printed circuit board. That is, to achieve the objective of being smaller, a driving IC and the electrical components are mounted on a film without the use of a traditional printed circuit. Thus, the COF process is usually employed to package a driving IC on a display panel.
Typically, a substrate of the film printed circuit board is made of polyimide (PI). An alloy layer is positioned on the substrate, and a copper metal layer is positioned on the alloy layer wherein the alloy layer is made of Ni—Cr alloy. A patterned photo resistant layer is formed on the copper metal layer by a lithography process, and then the copper metal layer, which is exposed, and the alloy layer under thereof are etched by a wet etching process. The wet etching process may employ a solution containing copper chloride, ferric ammine etc, as an etchant.
FIG. 1 is a sectional view of a conductive copper line formed by a traditional process. After the patterned photo resistant layer 106 is formed, the copper metal layer on the Pi substrate 100 is etched by the wet etching process to form a conductive copper line 104. A side effect of the wet etching process is that undercuts occur under the photo resistant layer 106, which makes the conductive copper line 104 to have a narrower top and a wider bottom. Generally, the thickness of the conductive copper line 104 is preferably between 6-12 μm, and the top width of the conductive copper line 104 should be more than half of the bottom width of the conductive copper line 104. If the top width of the copper line is too narrow, following processes, such as inner lead Au bump packaging and outer lead anisotropic conductive film (ACF) packaging processes, will be hard to perform. A way to increase the top width of the conductive copper line is to reduce the thickness of the conductive copper line, but if the thickness of the conductive copper line is too thin, the conductive copper line will be broken when the film printed circuit board is bent.
A solution to the problem mentioned above has been developed. FIG. 2A and FIG. 2B are schematic views of a traditional process for preventing the top width of the copper line from being too narrow. As illustrated in FIG. 2A, an alloy layer 202 and a copper metal layer 204 are positioned on a PI substrate 200 in order. A patterned photo resistant layer 206 is formed on the copper metal layer 204, and the patterned photo resistant layer 206 has openings 207. Another copper metal layer 208 is formed in the openings 207 by an electroplating process, and then, as illustrated in FIG. 2B, the patterned photo resistant layer 206 is removed, and thus the copper metal layer 208 protrudes from the copper metal layer 204.
As illustrated in FIG. 2C, the copper metal layer 208, copper metal layer 204 and alloy layer 202 are etched by a wet etching process until the PI substrate 200 is exposed. Parts of the copper metal layer 204, which are positioned in the opening 211 (illustrated in FIG. 2B), are completely removed and the other parts of the copper metal layer 204 which are positioned under the copper metal layer 208 partially remain. The side-walls of the conductive copper lines 210 have a recess caused by the wet etching process with the top width of the copper metal lines 210 maintained, a characteristic of isotropic etching.
The solution mentioned above seems effective but still has serious problems. As illustrated in FIG. 2A, the alloy layer 202 usually exists and is positioned under the copper metal layer 204, and the alloy layer 202 is typically made of Ni—Cr alloy. It is more difficult to etch Ni—Cr than copper when forming conductive copper lines 210 by the wet etching process. Thus, if the time of the wet etching process is not enough, the alloy layer 202, which is positioned between the conductive copper lines 210, will not be removed completely such that a micro-short problem will occur between the conductive copper lines 210; if the time of the wet etching process is increased to assure that the alloy layer 202, which is positioned between the conductive copper lines 210, can be removed completely, undercuts due to the over-etching the alloy layer 202 will occur under the conductive copper lines 210, such that the base of the conductive copper lines 210 will be weak. That is, although the solution mentioned above is able to improve the top width of the conductive lines, undercuts may still occur. Obviously, this solution cannot solve all problems related to manufacturing film printed circuit boards effectively.